The present invention relates generally to apron drafting systems for textile fibers, and more particularly to an apron drafting cradle having improved sliver control and maintenance features for detecting drafting irregularities. Three roll drafting systems are typically used in spinning and roving frames. The three roll system includes three pairs of upper and lower roll sets at spaced intervals. The sliver is fed initially to the back rolls, then to a set of middle rolls, and then to a set of front rolls from which the drafted sliver passes to a bobbin on which the sliver are roving is wound. Each of the sets of rolls is driven successively at a faster speed than the previous set which constitutes a draft ratio between the sets of rolls. The speed ratio between the middle set of rolls and the back rolls defines the back draft ratio and the speed ratio between the front set of rolls and the middle set of rolls is the front draft ratio. In each set of rolls, the top roll is freely rotatable and is driven by the bottom roll.
In order to exercise better control over the fibers in the front drafting zone, it has been typical to provide a drafting apron device having flat rubber aprons between which the fibers are drafted. The upper apron travels about the top roll and the lower apron travels about the bottom roll. The middle drafting apron may be one of two types. First there is the split cradle design in which the structures for mounting the aprons are separate and the other is a one-piece drafting apron cradle in which the upper and lower aprons and rolls are mounted on a single cradle. Examples of the one-piece apron drafting cradle are shown in U.S. Pat. Nos. 4,274,179 and 3,872,546. The desirability of exerting positive control over the fibers during drafting are known as discussed in U.S. Pat. Nos. 4,727,626 and 4,391,021. U.S. Pat. No. 4,727,626 is concerned with adjusting the nip points of a drafting zone to compensate for different fiber lengths in the sliver being drafted. In this manner, effective control may be had for different types of fibers to avoid fiber breakage and other drafting problems. U.S. Pat. No. 4,391,021 seeks to provide better fiber control in the back drafting zone. In the conventional one-piece cradle design, the upper and lower aprons pass over guide pins before returning to the respective top and bottom rolls which are rotating. Generally, the guide pin is arranged as close as possible to the nip of the front set of drafting rolls so the fibers can be guided a maximum distance between the two aprons in the front drafting zone. U.S. Pat. No. 4,391,021 proposes to place guide pins at the front and the back of the aprons while moving the top and bottom rolls towards the center of the assembly. However, fibers may still gather excessively between the belts causing the belts to separate and lose control over the sliver between the apron pins at the ends of the aprons. In addition, platforms may be inserted beneath the lower apron so that a generally S-shaped deflection may be imparted to the fibers which is said to increase control. In essence, the entire nip between the top and bottom rolls of the apron drafting system is taken up by the aprons leaving little, if any, room to accommodate variances in the thickness of sliver travelling between the rolls. This arrangement is said to provide a nip between the middle set of rolls toward both the nip of the back rolls and the front rolls to provide increased control over drafting in the back drafting zone.
The problem of maintaining the surfaces of the upper and lower aprons in contacting relationship in order to maintain control of the slivers drafted between the aprons remains to be a problem to which considerable attention needs to be given. Broken fibers and other fiber irregularities can build up between the upper and lower aprons causing the apron surfaces to, at least partially, lose contact resulting in loss of control over the slivers being drafted. Further, it has been difficult to detect the presence of fiber accumulation and other fiber irregularities occurring between the drafting aprons which is a critical area in the drafting process. Due to the obscurity of the location of the drafting apron assembly, particularly the one-piece cradle type used on a roll stand as shown in U.S. Pat. No. 3,872,546, it is difficult to observe malfunctioning.
Often, extraneous fibers can accumulate on the drafting aprons and be conveyed between the aprons causing fiber build-up and loss of sliver control. U.S. Pat. No. 4,274,179 proposes a clearer pad for maintaining the top apron clear of fibers. However, this device is mounted in a space consuming manner and may have only a limited biasing force against the apron to keep it clean. The attachment does not provide for mounting to the bottom for clearing the lower apron.
Accordingly, an object of the invention is to provide a one-piece drafting apron having increased sliver control between the drafting aprons.
Another object of the invention is to provide a drafting apron assembly having better control over the fibers between the drafting aprons.
Another object of the invention is to provide an apron drafting cradle having improved means for clearing extraneous fibers from the surfaces of the upper and lower aprons.
Another object of the invention is to provide a one-piece sliver drafting cradle in which inspection of the area between the drafting aprons may be had to detect the build-up of extraneous fibers and other fiber irregularities which adversely effect the drafting process